1. Field of the Invention
The present invention relates to a method for manufacturing lepidocrocite (.gamma.-FeOOH). More specifically, it relates to a method for manufacturing lepidocrocite suitable as a starting material in the manufacture of magnetic powders for magnetic recording media, such as audio tapes, video tapes, magnetic disks and magnetic cards.
2. Description of the Related Art
Generally, magnetic iron oxide powders for magnetic recording media such as audio tapes, video tapes, magnetic disks and magnetic cards are produced from goethite (.alpha.-FeOOH) or lepidocrocite (.gamma.-FeOOH) as a starting material. .alpha.-FeOOH or .gamma.-FeOOH is subjected to treatments such as calcination (for dehydration and inter-sintering), reduction, and oxidation to obtain maghemite (.gamma.-Fe.sub.2 O.sub.3) which are needle-shaped (acicular) magnetic iron oxide powders. The .gamma.-Fe.sub.2 O.sub.3 powders may be subjected to a further treatment of cobalt modification, to provide cobalt-coated .gamma.-Fe.sub.2 O.sub.3 powders (Co-.gamma.-Fe.sub.2 O.sub.3). Further, acicular .alpha.-FeOOH or .gamma.-FeOOH is subject to a treatment such as hydrogen gas reduction while maintaining the acicular shape of the starting material, to produce acicular magnetic metal iron powders.
In the above cases, the magnetic properties of the resultant magnetic powders depends on the characteristics of the starting material. Therefore, to obtain magnetic powders suitable for magnetic recording media, it is necessary to use a starting material having narrow particle size distribution, good shape and crystal structure.
Heretofore, magnetic powders produced from .gamma.-FeOOH (lepidocrocite) as the starting material provide magnetic recording media, as the final products, such as an audio tape and a video tape which have and excellent magnetic orientability, squareness ratio, and print-through level. But the particle size distribution of the magnetic powders produced from .gamma.-FeOOH is so wide that the switching field distribution of the magnetic recording media as the final products become high. As a result, at present, .alpha.-FeOOH (goethite) is nearly always used as the starting material for the production of .gamma.-Fe.sub.2 O.sub.3.